Characterization of the isolated active layer of a NF membrane by electrochemical impedance spectroscopy

• Dielectric and electric properties determined on the isolated active layer of a NF membrane.
• Very low contribution of the active layer to the overall membrane impedance.
• Dielectric constant inside pores of the active layer is smaller than its bulk value.
• Pore conductivity (κp) may exceed the bulk conductivity.

The dielectric and electric properties of the isolated active layer of an NF membrane were determined by electrochemical impedance spectroscopy (EIS). To our knowledge, this was the first attempt aimed at assessing the above properties from measurements performed directly on the active layer alone. Experiments were conducted with Desal DK membrane and various electrolyte solutions (KCl, KI, LiCl, NH4Cl, CaCl2 and BaCl2) at 10−2 mol m−3. The comparison of impedance spectra obtained for the whole membrane and active layer alone revealed very low contribution of the active layer to the overall membrane impedance. The dielectric constant (εp) and conductivity (κca) inside the pores of the active layer were evaluated from capacity and resistance measurements. The dielectric constant inside the pores was found to be smaller than its bulk value, irrespective of the electrolyte used. The reduction in εp was attributed only to confinement effect. It was also shown that pore conductivity exceeded the bulk conductivity. This increased conduction inside the pores could have been due to the excess of counter-ions inside the pores resulting from fixed charges on the pore walls. The present study shows that the dielectric constant inside the pores of the active layer of NF membranes can be determined independently from nano-filtration measurements and without any ambiguity, provided that EIS measurements are performed directly on the isolated active layer.

Dielectric constant inside the nanopores of the active layer of the DK membrane after immersion in various salt solutions at 10−5 M. This parameter was calculated using cylindrical parallel pore model. Tests were repeated on different samples.Figure optionsDownload high-quality image (100 K)Download as PowerPoint slide